As one of the high-speed imaging methods of the MRI apparatus, there is an imaging using a gradient echo method. In the pulse sequence of the gradient echo method, a high frequency magnetic field pulse for excitation is applied, and then a dephasing gradient magnetic field for diffusing the phase of the spin is applied in the reading direction before measuring a magnetic resonance signal. Then, a rephasing gradient magnetic field is applied to generate an echo. By measuring a signal for a predetermined sampling time while the rephasing gradient magnetic field is being applied, data before and after the peak of the signal including the peak can be acquired.
In the case of imaging using this gradient echo method, it is possible to shorten the imaging time significantly since an echo time TE (time for which the peak of a signal occurs) is short compared with a pulse sequence in a spin echo system in which a high frequency magnetic field pulse for spin inversion is used. However, since the application of the dephasing gradient magnetic field is essential, there is a limitation in shortening of the echo time TE.
On the other hand, a technique has been proposed which measures a signal within a shorter echo time by measuring a signal from rising of a gradient magnetic field without using a dephasing gradient magnetic field as a readout gradient magnetic field and using a half-wave high frequency magnetic field pulse as a high frequency magnetic field pulse which excites the spin (Patent Documents 1 and 2). This method is called a UTE measurement and can shorten the TE further. Accordingly, applications to imaging of a tissue with a short transverse relaxation time T2 which is difficult to image with a conventional MRI, for example, bone tissue and the like are expected.
In the UTE measurement, a radial scan is performed without using a phase encoding gradient magnetic field. In the radial scan, a plurality of signals is measured while changing the gradient magnetic field strength of a readout gradient magnetic field in each direction which is formed by gradient magnetic fields in two or three axial directions. The measurement data acquired in this way is arrayed radially from the origin of the k space since the measurement data is collected toward the base from the peak of the signal. In order to array the radial data in the grid of the k space, processing called gridding is necessary. The gridding is a processing of transforming the coordinates of measurement data, which is determined by the gradient magnetic field strength of the readout gradient magnetic field in each axial direction, into the coordinates of a grid point of the k space formed by a square or cubic lattice. By setting the measurement data acquired by nonlinear measurement as k space data by gridding, image reconstruction based on an operation, such as fast Fourier transform, becomes possible, for example.
In the gridding, the coordinates of raw measurement data are determined on the basis of the calculated value (application timing and strength of the gradient magnetic field set in the apparatus) of an applied gradient magnetic field. However, a gradient magnetic field actually applied deviates slightly from the calculated value of the gradient magnetic field due to the characteristics of a gradient magnetic field coil, shift of the control timing of an apparatus, and the like. For this reason, the k space data after gridding based on the calculated value includes an error, deteriorating the image quality when this is reconstructed.
The problem of gridding resulting from the difference between the set value (theoretical value) of the gradient magnetic field and the gradient magnetic field strength actually applied occurs not only in the UTE measurement but also in the radial scan. The inventor of this application proposes a method for solving this problem. This method is to calculate the shift amount for correcting the peak position (that is, a position which becomes the origin of the k space) of an echo signal using a plurality of echo signals.